CellCept and Myfortic already carry a "black box" warning, the FDA's sternest warning, about the risk of miscarriage during the first trimester and birth defects in babies born to women who take the drugs while pregnant.

Those warnings have been in place since November 2007, which is when the FDA also put the drugs in its pregnancy category "D." That classification means that there is evidence of human fetal risk, though the drugs' potential benefits in pregnant women may outweigh that risk.

The FDA says it's aware of reports of babies born with serious birth defects following exposure to MMF, the active ingredient in CellCept. Most of those babies were born to women who had gotten organ transplants, but some were using the drug off-label to treat immune disorders such as lupus or rheumatoid arthritis, according to the FDA. Myfortic's active ingredient, MPA, is chemically related to MMF.

The FDA is working with the drugs' makers to mitigate the risks of fetal exposure. Meanwhile, the FDA has this advice for women of childbearing age:

Tell your doctor if you are planning a pregnancy and using MMF (the active ingredient in CellCept) or MPA (the active ingredient in Myfortic).

You must use two forms of effective birth control if you are taking MMF or MPA, unless you choose abstinence as your only form of birth control.

You must start using two forms of birth control four weeks before you begin taking MMF or MPA and continue until six weeks after you stop taking the medicine.

CellCept is made by the drug company Roche. "As patient safety is of the utmost importance to Roche, we continue to work closely with the FDA to develop and implement means to mitigate the risks of fetal exposure," Roche spokesman Chris Vancheri tells WebMD in an email.

Myfortic is made by the drug company Novartis. A Novartis spokesperson wasn't available for comment in time for publication.